Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor (“directly driven”) either directly or through the use of a gearbox.
Wind turbines generally comprise a tower upon which a nacelle is mounted. A rotor comprising a hub and a plurality of blades may be rotatably mounted in said nacelle. To capture enough energy it is necessary to use large blades.
Given the important variability of the wind speed, the operation of the wind turbine is sometimes controlled by varying the angle of each blade about its axis for maintaining the wind turbine rotational speed. In this case, the blade is attached to the hub by means of a connection with a degree of rotational freedom, e.g. a bearing. This arrangement is very expensive but provides a variable pitch wind turbine, which may be necessary to significantly improve the performance of the wind turbine. ‘Pitch’ refers to turning the angle of attack of the blades in response to variations in the wind speed to adjust the rotation speed of the wind turbine.
At low or medium wind speeds, which yield a power below the rated power of the generator, the angle of attack can be set at its maximum, although it can be smaller to help the wind turbine to accelerate faster. Above the rated wind speed, the pitch is controlled to keep the generator power at the rated power by reducing the angle of attack of the blades; besides, in this way the various parts of the wind turbine are protected from the damage that can be inflicted upon them by such strong winds.
However, the pitch control does not allow one to achieve an adaptation to the local variable flow conditions, i.e., to the varying flow conditions along and across the whole blade. Moreover, the pitch control involves rotating the whole blade as a block around the pitch axis, which implies overcoming a high inertia, whereby the time of response may be rather long and power consuming.
It is known to divide a wind turbine blade in several longitudinal segments that can pivot with respect of each other in order to define an aerodynamically-enhanced twisted blade over a wide range of wind speeds. WO2005 064156A1 discloses “fractioning the blade into at least three segments of appropriate aerodynamic shape, each segment being divided along the span of the blade and being adapted to rotate independently of each other about an axis substantially parallel to the spanwise axis of the blade”. But this arrangement implies a lack of continuity between adjacent segments, which may produce turbulence and noise.